Answer: We're used to thinking of insomnia and poor sleep as being a product of depression. More recent studies are indicating the arrow may point in the opposite direction.

That is, that insomnia which lasts for long periods of time may predispose individuals to developing new depressive episodes. With sleep apnea syndrome, which features poor sleep, that could very easily lead to depression at a much higher rate than individuals who do not have sleep apnea syndrome.